GPS is good for phones and car nav systems, where being a few feet off (or a dozen) doesn't matter because terminal guidance uses other procedures. Note that car lane-keeping systems do NOT use it; they use other indicators, like lane lines. Surveyors use it, but need to occupy a site for a while to get enough satellites to achieve the needed precision and sometimes refer to a signal at a known location on the ground. IOW it's good but not good enough for reliable, real-time precision unless you're military. Adding those other features to correct GPS probably add a bit of cost & complexity to PTC, assuming that's happening.

Congresscritters who see their cell phone telling them what block they're on, and how to get to the address where the next fundraiser is at, think GPS is great. Those who deal with the real world know that it's good and useful, but not perfect.

IOW, there will be times when GPS isn't good enough and a "pilot" must take over. Even on automated transit lines, there's usually a "train attendant" as the ultimate backup using the Mark I Eyeball, even if the only available control is a big red STOP button.